This invention relates to a glass substrate for a mask blank without fine convex surface defects on the surface of the substrate, a method of producing the same, a mask blank using the glass substrate, a method of producing the same, a transfer mask, a method of producing the same, and a method of producing a semiconductor device.
Following the recent development of a higher-density and higher-accuracy VLSI device, a glass substrate for an electronic device, such as a glass substrate for a mask blank, is required to have good flatness and less surface defects. Such demand becomes more and more strict year by year.
In order to reduce the surface roughness of the glass substrate for a mask blank, a precision polishing process is proposed, for example, as disclosed in Japanese Unexamined Patent Publication No. H1-40267 (JP 1-40267 A). The precision polishing process includes a polishing step using a polisher comprising cerium oxide as a main component and a final polishing step using colloidal silica. It is noted here that commercially available colloidal silica has a pH value between 9 and 10.5 in view of the stability. However, when it is diluted, the pH value is lowered. According to the above-mentioned publication, it is therefore preferable to add inorganic alkali, such as NaOH and KOH, or organic alkali, such as amine, to colloidal silica so as to increase the pH value up to 11. Addition of alkali is also advantageous because alkali has an effect of etching the glass and such effect is synergistically exhibited.
The present inventors diligently and thoroughly examined about whether or not the surface of the glass substrate subjected to the final polishing step using colloidal silica increased in pH value as mentioned above satisfies high-level requirement for the flatness and the surface defects as the above-mentioned recent demand. As a result, it has been found out that convex protrusions having a height on the order of several nanometers and a dimension between several tens nanometers to 2000 nanometers are often formed on the surface of the glass substrate subjected to the final polishing step in the above-mentioned manner. The convex protrusions have such a small height and could not be confirmed by traditional visual inspection. The presence of such convex protrusions could not be confirmed without a defect inspection system which has been developed in order to confirm a defect-free surface which meets the recent demand of a high level.
When a thin film is formed on the convex protrusions and a mask blank and a transfer mask are produced, the dimension of the convex protrusions is enlarged. Therefore, even if the substrate itself meets the demand for 0.3 μm defect free, 0.1 μm defect free, and 0.05 μm defect free as a next-generation substrate, the mask blank and the transfer mask using the substrate may be found defective in defect inspection.
In case where a phase shift mask blank or a phase shift mask is produced by the use of the glass substrate with the convex protrusions having a height on the order of several nanometers, change in phase angle due to presence of the convex protrusions becomes greater to cause phase defects as an exposure wavelength of exposure light becomes shorter. As the exposure wavelength becomes shorter, the influence of the convex protrusions becomes greater. The problem of the phase defects is serious in next-generation lithography using an ArF excimer laser, an F2 excimer laser, or an EUV (Extreme Ultra Violet) light source as an exposure light source. For example, it is assumed that the convex protrusions have a height of 5 nm. If the exposure light is ArF having the wavelength of 193 nm, the change in phase angle is 4.6 degrees. If the exposure light is F2 having the wavelength of 157 nm, the change in phase angle is 5.7 degrees. Consideration will be made about the case where an EUV reflective mask blank or an EUV reflective mask is produced by the use of the glass substrate with the convex protrusions having a height on the order of several nanometers. If the convex protrusions have a height of 5 nm, the change in phase angle exceeds 20 degrees when the exposure wavelength is 13.5 nm. This change in phase angle results in increased error and degradation of CD (Critical Dimension) characteristics, which is not a negligible problem.